Silica, found in ordinary sand, is an abundant inorganic material having silicon dioxide (SiO.sub.2) as a basic structural unit. One type of silica particles is termed pyrogenic, or "fumed silica." Fumed silica typically has a particle size from about 2-20 .mu.m, and is most commonly prepared from a vapor phase. For example, silica can be vaporized at about 2000.degree. C. in the presence of a reducing agent (e.g., coke) to form SiO, which can be oxidized to form particulate silica. Other methods of producing fumed silica include, for example, oxidation of silicon tetrachloride (SiCl.sub.4) at high temperatures or burning SiCl.sub.4 in the presence of methane or hydrogen.
One growing commercial application for fumed silica is as the major component in porous (e.g., usually having pores in the 1-1000 nm range) thermal insulation. For example, fumed silica is used in many applications in which high-temperature thermal insulation is required, such as electric stove tops, steam pipes, industrial ovens and furnaces, and elevator fire protection systems. In these and other thermal insulation applications, the fumed silica is typically blended with infrared opacifiers, such as, for example, TiO.sub.2 (i.e., titania), carbon black, or zirconium silicate. In addition, the fumed silica is often also blended with fibers (usually in the form of glass, plastic, and/or ceramic) in order to enhance the toughness of the insulation that is ultimately formed. The blend of fumed silica, infrared opacifiers, and, possibly, fibers, is compacted to a target density by applying pressure, usually uniaxially. This increase in density is sought in order to improve the mechanical strength of the insulation.
A significant drawback with known methods of producing fumed silica-based thermal insulation is that, after the pressure for compacting the material is released, the materials suffer from "volume springback" (i.e., they expand in the direction from which the pressure was applied). Not only does this result in a lower actual density, as compared with the target density to which the material was compacted, but it also causes a concomitant decrease in mechanical strength, as well as dimensional uncertainty, delamination, and cracking in the compact.
Another drawback with known methods of producing fumed silica-based thermal insulation relates to mechanical strength. Because many thermal insulation applications have specific mechanical strength requirements that must be met, known methods of producing fumed silica-based thermal insulation require adjusting the density until the desired mechanical strength is exhibited by the insulation. In this regard, although thermal performance is fairly independent of density, the mechanical strength of the insulation is dependent primarily on density in known methods of producing fumed silica-based thermal insulation. Since the fumed silica is the most significant raw material expense in this type of insulation, merely increasing the density to achieve a desired mechanical strength is undesirable because of the high costs associated therewith.
From the foregoing, it will be appreciated that there is a need for an improved method of compacting a fumed metal oxide-containing composition in which the amount of volume springback exhibited by the compact is reduced. It will also be appreciated that there is a need for an improved method of compacting a fumed metal oxide-containing composition in which the mechanical strength of the resulting compact is enhanced at a given density. It is an object of the present invention to provide such a method.